Why is my Smooth tool causing the surface of my sculpt to be reflective?
The culprit is actually the Solidify modifier. When i applied the mirror I was under the impression that i needed to use solidify to make the mirrored geometry physical.

Solidify will make an internal geometry. when sculpting, i would only be hitting the upper mesh and it would clip through the lower.

Lesson learned: do NOT solidify objects you intend to add to a sculpt.